DESCRIPTION

The kernel uses a radix tree structure to manage routes for the
networking subsystem. The rtalloc() family of routines is used by
protocols to query this structure for a route corresponding to a
particular end-node address, and to cause certain protocol- and
interface-specific actions to take place.
When a route with the flag RTF_CLONING is retrieved, and the action of
this flag is not masked, the rtalloc facility automatically generates a
new route using information in the old route as a template, and sends an
RTM_RESOLVE message to the appropriate interface-address route-management
routine (ifa->ifa_rtrequest()). This generated route is called cloned,
and has RTF_WASCLONED flag set. RTF_PRCLONING flag is obsolete and thus
ignored by facility. If the RTF_XRESOLVE flag is set, then the
RTM_RESOLVE message is sent instead on the route(4) socket interface,
requesting that an external program resolve the address in question and
modify the route appropriately.
The default interface is rtalloc(). Its only argument is ro, a pointer
to a “struct route”, which is defined as follows:
struct route {
struct sockaddr ro_dst;
struct rtentry *ro_rt;
};
Thus, this function can only be used for address families which are
smaller than the default “struct sockaddr”. Before calling rtalloc() for
the first time, callers should ensure that unused bits of the structure
are set to zero. On subsequent calls, rtalloc() returns without
performing a lookup if ro->ro_rt is non-null and the RTF_UP flag is set
in the route’s rt_flags field.
The rtalloc_ign() interface can be used when the default actions of
rtalloc() in the presence of the RTF_CLONING flag is undesired. The ro
argument is the same as rtalloc(), but there is additionally a flags
argument, which lists the flags in the route which are to be ignored (in
most cases this is RTF_CLONING flag). Both rtalloc() and rtalloc_ign()
functions return a pointer to an unlocked structrtentry.
The rtalloc1() function is the most general form of rtalloc() (and both
of the other forms are implemented as calls to rtalloc1). It does not
use the “struct route”, and is therefore suitable for address families
which require more space than is in a traditional “struct sockaddr”.
Instead, it takes a “struct sockaddr *” directly as the sa argument. The
second argument, report, controls whether RTM_RESOLVE requests are sent
to the lower layers when an RTF_CLONING or RTF_PRCLONING route is cloned.
Ordinarily a value of one should be passed, except in the processing of
those lower layers which use the cloning facility. The third argument,
flags, is a set of flags to ignore, as in rtalloc_ign(). The rtalloc1()
function returns a pointer to a locked structrtentry.
The rtfree() function frees a locked route entry, e.g., a previously
allocated by rtalloc1().
The RTFREE() macro is used to free unlocked route entries, previously
allocated by rtalloc() or rtalloc_ign(). The RTFREE() macro decrements
the reference count on the routing table entry (see below), and frees it
if the reference count has reached zero.
The preferred usage is allocating a route using rtalloc() or
rtalloc_ign() and freeing using RTFREE().
The RT_LOCK() macro is used to lock a routing table entry. The
RT_UNLOCK() macro is used to unlock a routing table entry.
The RT_ADDREF() macro increments the reference count on a previously
locked route entry. The RT_REMREF() macro decrements the reference count
on a previously locked route entry.

RETURNVALUES

The rtalloc(), rtalloc_ign() and rtfree() functions do not return a
value. The rtalloc1() function returns a pointer to a routing-table
entry if it succeeds, otherwise a null pointer. Lack of a route should
in most cases be translated to the errno(2) value EHOSTUNREACH.

SEEALSO

HISTORY

The rtalloc facility first appeared in 4.2BSD, although with much
different internals. The rtalloc_ign() function and the flags argument
to rtalloc1() first appeared in FreeBSD 2.0. Routing table locking was
introduced in FreeBSD 5.2.

AUTHORS

This manual page was written by Garrett Wollman, as were the changes to
implement RTF_PRCLONING and the rtalloc_ign() function and the flags
argument to rtalloc1().